Concrete in Australia Vol 37 No 2 25
Feature technical paper: hiGh StrenGth cOncrete (peer reVieWeD)
1.2 .2 Failure mode 2: Spalling due to restrained
thermal dilatation
Spalling due to restrained thermal dilatation was identified by
Bazant (1997) and later adopted by Ulm et al (1999a) and
Nechnech et al (2002). This failure mode considers that the
spalling results from restrained thermal dilatation close to the
heated surface, which leads to compressive stresses parallel to
the heated surface.
These compressive stresses are released by brittle fracture
of concrete, i.e . spalling. Due to the volume expansion of a
growing crack, and the slowness of release of additional water
into the crack, the pressure in the crack must rapidly decay
after the crack begins to open. As a result, the pore pressure
can play only a secondary role as far as the growth of a larger
crack is concerned. The pore pressure may affect the onset of
instability in the form of explosive thermal spalling (Figure 6).
Figure 7 shows a slab after a fire test exposing only the middle
region of the slab to fire, and illustrates this failure mode
(Hertz, 2003).
1.2.3 Failure mode 3: Thermal incompatibilities
between cement paste and aggregates
When subjected to increasing temperature, the cement paste
initially expands and when it is heated beyond about 300°C,
it starts to rapidly contract. Figure 8 shows the behaviour of
four different cement pastes reported in the literature. Due
to thermal gradients in concrete, parts which are still under
300°C would be experiencing expansion while the other
parts which are more than 300°C would be experiencing
contraction. This competition between simultaneous
expansionand contraction damages the concrete matrix. This
behaviour depends on the type of cement binders used.
With increasing temperatures, most types of aggregates
undergo expansion. Aggregates typically occupy about 60
Figure 6. Spalling due to restrained thermal dilatation.
Figure 8. Contraction of Portland cement paste at high temperatures: (a)
Phileo (1958); (b) Harada et al (1972); (c) Cruz and Gillen (1980); (d)
Crowley (1956).
Figure 9. Thermal incompatibilities in concrete at high temperatures.
Figure 7. Spalling due to restrained thermal dilataion.